1. Field of Use
This invention relates generally to a device for testing and calibrating a known type of electrical instrument which is used for measuring the moisture content of a sample of grain or other particulate material.
2. Description of the Prior Art
U.S. Pat. No. 2,693,575, issued Nov. 2, 1954 to Greenwood et al for "Moisture Determining Device for Materials" discloses an electrical instrument which is used for measuring the amount of moisture in a sample of grain or other particulate material. This instrument, which operates on electrostatic principles, generally comprises a test cell for holding a material sample of known type and size whose moisture content is to be determined and further comprises a comparative capacitance unit. The test cell comprises a cylindrical metal container open at its upper end and closed at its bottom end by an insulating disc and a cylindrical metal central electrode (insulated on the outside of a plastic sleeve) concentrically mounted within the container on the insulating disc in spaced relationship with the container. The test cell is, in effect, a capacitor and the container and the central electrode serve as the electrodes which are part of a resonant electrical circuit of the comparative capacitance unit. In operation, the resonant circuit is first balanced with the test cell empty. The material sample is then added to the cell, thus increasing its capacitance and the circuit is then rebalanced by means of a calibrated standard condenser and the change in capacity as indicated by this condenser, and a suitable indicator, is then correlated to the moisture content. In practice, moisture determining instruments of this character are accurately calibrated during manufacture but are subject to miscalibration during use as the sensitive electrical components therein age or deteriorate during use. Accordingly, a means in the form of a manually adjustable trimmer condenser is provided to enable the instrument to be accurately recalibrated periodically. Heretofore, testing of the instrument for accuracy and recalibration required the use of a controlled grain sample which took the form of a measured amount of grain or other material having known dielectric characteristics and moisture content. Such samples are available in sealed containers from the U.S. government through specified channels of distribution which exist in the trade, and recalibration using such samples is carried out on a periodic basis. While this method of testing and recalibrating moisture meters is very accurate and reliable, the procedures for obtaining the controlled samples are cumbersome and no testing or calibration was heretofore possible unless the controlled sample was available. U.S. patent application Ser. No. 729,128, filed Oct. 4, 1976, which was allowed on Aug. 29, 1977 as U.S. Pat. No. 4,066,951 on Jan. 3, 1978 and assigned to the same assignee as the present application, discloses a calibration apparatus which comprises a dielectric member for slideable insertion between the container wall and the electrode to a predetermined distance to effect a change in field intensity, means for holding the dielectric member in a desired position in the test cell, a pointer detachably connectable to the test cell, and a graduated scale on the dielectric member for cooperation with the pointer to indicate visually a numeric value which corresponds to a numeric value which will appear as the visual meter readout if the instrument is calibrated correctly.